The exhaust gas coming from a vehicle, and in particular a diesel-engined vehicle contains a certain amount of pollution, and in particular particles of soot. Recent anti-pollution standards applied to this type of engine require such particles of soot to be eliminated almost entirely from exhaust gas. This elimination is generally performed by means of a particle filter which tends to become clogged progressively while the engine is in operation, because of the presence of these particles. In order to clean the filter, it is necessary to burn the soot.
For this purpose, various additives are added to diesel fuel such as organo-metallic compounds which act as combustion catalysts for soot particles.
More generally, it is possible to add various additives to any type of fuel depending on the kind of vehicle engine, depending on requirements specified by the manufacturer, and depending on the need to comply with various pieces of anti-pollution legislation.
As a general rule, such additives are mixed in with the fuel during the industrial process of preparing the fuel, in refineries where petroleum fuels are produced.
Nevertheless, consideration has more recently been given to providing not only a fuel tank within a motor vehicle, but also a separate additive tank together with systems for introducing a measured quantity of the additive into the fuel tank. Such systems serve to control the quantity of additive that is added (generally in such a manner as to give a volume of additive that corresponds to the volume of fuel added to the tank).
Thus, document FR 2 718 795 discloses a motor vehicle fuel feed system that includes means for introducing additive into the fuel. The installation described in that document comprises firstly a fuel tank provided in conventional manner with a suction module designed to direct said fuel towards the site where it is used, and secondly an additive tank into which a suction module dips, which module is connected to an injector via an introduction duct. The injector is mounted on the top wall of the fuel tank and serves to inject additive directly into the tank. In addition, excess additive directed to the injector is recovered in a return duct fitted with a pressure regulator.
Document EP-0 488 831 describes an installation comprising firstly a pump for injecting fuel into the cylinders of an engine which is connected to a fuel tank via a suction pipe, and secondly an additive injector connected to an additive tank and to an electronic control unit. The injector enables additive to be injected directly into said suction pipe.
Simplified installations are also known in the prior art in which the additive pump and injector are replaced by a metering pump.
Thus, document U.S. Pat. No. 4,621,593 describes an installation comprising an additive tank connected to a fuel tank via a pipe having an electric pump mounted therein. The electric pump enables a measured quantity of additive to be introduced directly into the fuel tank. It is actuated by a control unit servo-controlled to a fuel gauge in the tank. Document DE 3 626 419 describes an installation that is similar except that the pump delivers the additive into a feed pipe for the fuel tank. Finally, document EP 0 269 228 discloses a pump which delivers the additive into the fuel return pipe that serves to return excess fuel from the engine to the fuel tank.
Nevertheless, those prior art systems require sufficient space to be available around the fuel tank for receiving the additive tank and the associated pumping means. Unfortunately, this space is not always available.